Engines do not need break in periods like they used to, even te manual says so. To me there is some common sense, give everything a few miles then have at it. A lot of old wives tales out there, same goes for the 3k oil change, there is nothing from manufacture, experts, oil experrts to point to this, but you still have the peace of mind guys out there.
GM has an interest in ensuring these things are good for 100-150k, on average people are getting new cars around 7-8 years or around 100-120k miles. Me personally Im not going past 50-60k much less 100k, for me its the next guys problem.